Stable Ammonium-Ion Batteries in Diluted Electrolyte By Manipulation of H-Bonding Network Via Ethylene Glycol

Abstract

Aqueous rechargeable batteries (ARB) arose considerable interest for stationary energy storage due to low cost, inherent safety, and environmental friendliness.[1] Besides aqueous lithium-ion batteries, various kinds of aqueous batteries have been proposed, utilizing relatively inexpensive charge carriers either based on metal (Na+, K+, Ca2+, Mg2+, Zn2+, Al3+, etc.) or non-metal cations (H3O+, NH4 + etc.).[2] Among non-metallic ion batteries, aqueous ammonium-ion batteries (AAIBs) distinguish themselves by working in near neutral or mild acidic environment, refraining from severe electrode corrosion.[3] However, like other ARB systems, the energy density of AAIBs is limited by the narrow electrochemical stability window of the aqueous electrolyte. In addition, NH4 + ions hydrolyse in water resulting in decreased pH of the electrolyte which promotes the hydrogen evolution reaction (HER).[3] This means that increasing salt concentration may not be an effective strategy to enlarge the stability window of the electrolyte. In fact, according to a previous report, the cathodic stability of 25 m ammonium acetate (NH4Ac) is even slightly inferior than that of 1 M NH4Ac.[4] Developing alternative strategies to inhibit HER in diluted electrolytes is of uttermost importance.In diluted aqueous electrolytes, the continuous and complicated H bond network among water molecules favours fast ionic motion but also causes severe HER. Here, we propose ethylene glycol (EG) as electrolyte component to manipulate the H bond structure in diluted (1 m) NH4Ac solutions. FTIR results are used to study the H-bonding network as function of the EG:water ratio. The diluted aqueous electrolyte containing EG show no freezing point and wider electrochemical window. As a proof-of-concept, full cells composed of Prussian blue derivatives, PTCDI anode, and 1 m NH4Ac with the addition of EG are tested to verify the effectiveness of EG during battery operation. Improved cycling performance and higher coulombic efficiencies are demonstrated compared to those including 1 m NH4Ac in water.